The company has offered iPhone and iPad controller apps for years, which are free and offer a complete set of features. I doubt the company sold many hardware controllers at US$349 when you could buy an iPod touch for about half the price and accomplish the same thing.
When the Sonos system was first offered for sale 10 years ago, smartphones were new, and some kind of small controller was necessary. With the invention of the iPhone, and the later ability to have dedicated apps, it was natural for Sonos to offer full control that way. An iPad app followed, and the dedicated controllers have become more and more irrelevant.
It's a small chapter in the larger book of changes that smartphone apps have added to the way we entertain ourselves, communicate and control household items like cable boxes, DVRs, and even security and lighting systems.